Conventional drop ejector printing processes that apply ultraviolet (UV) curable gel inks often result in various image related defects such as, but not limited to, lines that resemble a corduroy or vinyl record-like appearance, streaking, pin-hole defects, line deletion, dot deletion, patch deletion, gloss non-uniformity, etc.
UV curable gel inks are desirable for ink jet printers because they remain in a solid phase at room temperature during shipping and have long term storage capabilities, among other reasons. In addition, problems associated with nozzle clogging as a result of ink evaporation with liquid ink jet inks are largely eliminated with UV curable gel inks, thereby improving the reliability of the ink jet printing. Furthermore, in phase change ink jet printers wherein the ink droplets are applied directly onto the final recording substrate (such as, for example, paper, transparency material, and the like), the droplets solidify immediately upon contact with the substrate, so that migration of ink along the printing medium is prevented and dot quality is improved.
Nevertheless, gel inks require some type of transformation such as curing to prevent them from running or smearing when printed onto a substrate and subjected to general handling. In addition, uncured gel inks stick to roller surfaces in print paths, making them unsuitable for many printing applications without some sort of transformation or curing.
The aforementioned image defects are often caused by an uneven distribution of ink in an image area in which the image should be smooth and uniform. Because the ink temperature drops after ejection, the ink freezes on contact with the substrate and an uneven distribution of ink on the image substrate may occur. The human eye can sometimes observe the uneven distribution as bands or lines in the direction of the substrate travel past the print head, missing portions of the image, or gloss-related defects, for example. This uneven distribution might be addressed by leveling the ink on the image substrate with a contact member, such as a roller, belt, or wiper, in an effort to normalize the ink distribution. Leveling also enables uniform gloss appearance for better image quality, and facilitates line growth to compensate for missing or weak jetting.
Gel inks have very little cohesive strength prior to curing. In addition, gel inks are typically designed to have good affinity to many different types of materials. What this means is that that conventional methods for flattening a layer of ink tend to fail with respect to gel inks, because the gel ink splits. As the splitting occurs, the gel ink leaves a significant portion of the image behind on the device that is trying to flatten it, such as a traditional fuser roll typically used in xerography processes.